I'm deploying a shiny app to shinyapps.io. It's designed to be used with desktop browsers. When accessing the app on a mobile device it changes to mobile view of course (which is cool but not useful in this case).

Or if you know a bit about HTML and CSS you can learn more about the bootstrap grid system that shiny uses here then set your own custom css classes on shiny div() functions to force a certain width across all devices i.e. class = "col-xs-6"